1. Field of Invention
This invention relates to a device for monitoring the condition of a nickel-cadmium battery.
2. Prior Art
Nickel-cadmium batteries, comprising a plurality of nickel-cadmium voltaic cells connected in series, possess a number of highly desirable characteristics that include a relatively high capacity to weight ratio, a relatively flat voltage to percent of discharge curve, good performance at low temperatures, the capability of delivering high amperage currents for engine starting and similar purposes, and a relatively high recharge recycling capacity. Moreover, the chemistry of the nickel-cadmium system is such that nickel-cadmium cells may be hermetically sealed which permits operation in any position and in inaccessible locations. All of these characteristics have made nickel-cadmium batteries particularly well suited for use in aircraft and similar environments.
Aircraft electric systems normally include a rechargeable storage battery (for example, a nickel-cadmium battery) and a constant potential battery charging apparatus (for example, an alternator and voltage regulator) connected to the battery for restoring and maintaining its charge. However, when a nickel-cadmium battery is connected to a constant potential source of charging current the battery is susceptible to a malfunction known as "thermal runaway" which is a condition that can destroy the battery if undetected and unchecked. That is to say, when the battery temperature exceeds about 160.degree. F, the battery voltage decreases as the temperature rises. Moreover, the temperature of the battery tends to rise when the battery is being charged by a charging current from a constant potential charging current source. If, for some reason, the battery temperature should rise above the critical value of about 160.degree. F., the voltage of the battery may be lowered to the point where the charge current drawn by the battery increases sharply. This results in a further increase in temperature accompanied by a further voltage drop and still higher current consumption. Thus, a vicious cycle is created which will destroy the cell if undetected and allowed to go unchecked.
When nickel-cadmium batteries were first installed in aircraft the only battery monitoring devices employed were the usual ammeter to indicate battery discharge and charge rates and voltmeter to indicate the voltage at the battery terminals. The need for an additional instrument to detect and warn the pilot of incipient "thermal runaway" immediately became apparent, and battery monitors for the detection of "thermal runaway" are now mandatory equipment in aircraft.
Conventional monitors for the detection of incipient thermal runaway comprise a sensor for measuring the temperature of the nickel-cadmium battery and means for alerting the pilot (for example, a buzzer) when the temperature of the battery exceeds a predetermined value. When the battery temperature exceeds the predetermined value and the alarm is sounded, the pilot must disconnect the battery from the battery charging equipment and land the aircraft as quickly as possible. However, if the alarm activation temperature is set low enough to give the pilot sufficient time to take appropriate action to prevent battery destruction, the device will often give a "false alarm" of incipient thermal runaway when in fact the condition of the battery is perfectly stable. In order to prevent such false alarms the alarm activation temperature must be set so high (that is, only a few degrees below the critical temperature) that the pilot will often have insufficient time to take action to prevent battery destruction. Accordingly, it will seem that conventional nickel-cadmium battery monitoring devices are inherently unsatisfactory and, to some degree, unreliable.
After an intensive investigation of the problems inherent in the design of a satisfactory and reliable monitor for nickel-cadmium batteries, we have found that battery temperature is but one of the critical indicators of the condition of nickel-cadmium batteries. Low battery voltage is an indication of battery failure (for example, a dead cell) or charging equipment malfunction or failure. High charging voltage is an indication of malfunction of the battery charging equipment, and the resulting high charging current can lead directly to high battery temperatures and "thermal runaway" as previously described. Perhaps the most important indication of incipient thermal runaway is an excessive increase in the charging current within a short period of time. None of the nickel-cadmium battery monitoring devices heretofore known is capable of measuring these critical battery parameters.
As a result of our investigation we have now devised a new nickel-cadmium battery monitoring device that continuously monitors the most critical battery parameters -- namely, the battery temperature, the battery voltage and the rate of change in the battery charging current. The magnitude of the battery temperature and voltage is continuously visually displayed to the pilot, and a visual and audible alarm alerts the pilot whenever the voltage of the battery falls below or exceeds a predetermined value, whenever the temperature of the battery exceeds a predetermined value or whenever the rate of increase in the charging current exceeds a predetermined value. The instrument of the invention employs solid state circuitry throughout and is sturdy and reliable in operation.